Rod pumping an oil or gas well by a positive displacement fluid pump which consists of a traveling valve, a working barrel and a standing valve among other parts is well known art within the oil and gas industry. Rod pumping is centuries old, traced back to the Roman Empire which utilized the pressurizing ability to draw and lift fluids, typically water, great distances vertically. Similar rod pumping installations were used to de-water underground mine shafts hundreds of feet deep during the 16th century. The following terms/definitions for common oil and gas industry terms, shall apply to this application.
OPERATING COMPANY: The owner of the right to drill or produce a well, or the entity contractually charged with drilling a test well and production of all subsequent wells which produce sellable petroleum products.
MAIN OBJECTIVE for OPERATING COMPANIES: is to produce as much sellable hydrocarbon product(s) with the lowest daily operating expense including human labor. Along with minimizing the annualized costs of lost production caused by well system failures in addition to the costs associated with repairing all well failures.
HYDROCARBON: A naturally occurring organic compound comprising hydrogen and carbon. Hydrocarbons can be as simple as methane [CH4], but many are highly complex molecules, and can occur as gases, liquids or solids. The most common hydrocarbons are natural gas, oil and coal.
ADSORPTION: The property of some solids and gases to attract and collect on their surfaces. Coal seams and methane gas are typically found together. Methane adsorbs to the surface of coal while under pressure caused from ground water acting on the coal seam or coal bed.
DELIQUIFICATION: Also referred to as “gas well dewatering”, is the general term for technologies used to remove water or liquids or condensates build-up from producing gas wells all of which hinder natural gas production.
PETROLEUM: is a complex mixture of naturally occurring hydrocarbon compounds found in rock. Petroleum can range from solid to gas, but the term is generally used to refer to liquid crude oil.
FIELD: An accumulation, pool, or group of pools of hydrocarbons in the subsurface or subterranean. A hydrocarbon field consists of a reservoir in a shape or formation that has trapped hydrocarbons and that is covered by an impermeable, sealing rock. Typically, industry professionals use the term with an implied assumption of economic size. Reservoir formation is a commonly used term to describe the type of ground in which the trapped pools of hydrocarbons are found and produced from.
PRODUCTION: The phase that occurs after successful exploration and development and during which hydrocarbons are removed from an oil or gas field and produced at the surface of the well for additional processing and resale. Producing an oil or gas well by artificial means is lifting the subterranean fluids which enter the bottom of the well bore to the surface. The lifting energy is called artificial lift. Producing a well is also called pumping a well, it is synonymous.
ARTIFICIAL LIFT: Any system that adds energy to the fluid column in a wellbore with the objective of initiating and improving production from the well. Artificial-lift systems use a range of operating principles, including rod pumping, electric submersible pumps (esp), progressive cavity pumps (pcp), gas lift, plunger lift, jet pumps (hydraulic lift) and several other lesser used techniques.
ROD PUMPING: Is the most common artificial-lift system used in land-based operations today. Over 70% of all oil wells are currently produced using this technique. The down-hole positive displacement pump is operated and controlled thru lifting and lowering the rod string via the surface unit during operation. Rod pumping is the only form of artificial lift capable of pumping a well virtually dry and not damage itself if operated correctly.
WELL BORE DATA: Is commonly used to describe the entire well installation. It is also called the well bore diagram. It includes all information related to the complete installation of the well in the ground. It includes casing and production tubing string information such as sizes, weights, and grades plus rod string information including quantities of, lengths of, sizes and quantities of each size and length as well as all coupling information along with grades of material types. The well data also covers pump setting depth, length, how straight or not straight the well was initially drilled, is the well deviated, is the well a directional along with all down-hole pump specifics such as API type, bore, stroke, pump clearance, materials of construction, types of valves, quantities of valves.
SURFACE UNITS: Are known by several different names such as; surface unit, nodding donkey head, pump jack, Lufkin plus others. The surface unit lifts and lowers the polished rod. Mechanically driven units use a counterbalance that reduces the high torque requirements during the lifting cycle. The counterbalance averages the peak torque requirements of the complete pump cycle to allow for a smaller prime mover. Mechanically counterbalanced surface units have tremendous amounts of moving inertia, rendering them very difficult to stop and hold a given position upon command, hence it is not feasible for them to stop and dwell at the top of the polished rod lifting cycle. Since they are by nature all mechanical it is impossible for them to vary their polished rod travel distances autonomously. They must use a means of mechanical or friction braking system applied via external force to stop and hold the polished rod stationary. The external control force to apply the brake is very commonly a human actuating a lever which in turn amplifies the human input force acting to set the braking device. Releasing the brake would be just the opposite. Some hydraulically powered surface units use high pressure nitrogen gas to act as the counterbalancing effect. Their counterbalance effect is temperature sensitive and changes based upon ambient temperature change. Their ability to stop and hold polished rod position is also severely compromised due to the manner in which the high pressure nitrogen gas is utilized to counterbalance the well.
POLISHED ROD: The polished rod fully supports the entire sucker rod string weight plus all fluid column loads plus part of the down-hole pump during lifting portion of the pumping cycle. The polished rod also seals the top of the wellhead preventing hydrocarbons from escaping.
SUCKER RODS: Very strong, solid rods that are mechanically threaded and torqued together to form a single unit, commonly called the rod string. The assembled rod string is completely supported by and below the polished rod. The rod string connects the surface unit to the pump at the bottom of the well installation.
ROD STRING: Also called sucker rod string, consists of the polished rod plus sucker rods and couplings threaded and torqued together completely supported by and below the polished rod. The rod string supports the fluid column weight for one half of the total pump stroke cycle as the surface unit lifts the rod string. The rod string is one entire elastic unit. It stretches and contracts based on forces applied to it along with load hanging below the polished rod. The rod string transfers surface energy to the down-hole pump for powering and controlling the pumping action including the rate of discharge for the down-hole pump. The rod string mechanically lifts the traveling valve of the down-hole pump.
PRODUCTION TUBING STRING: Also called the tubing string and the conduit from which subterranean fluids are artificially lifted to the surface. The tubing string is made up of sections of tube and couplings which are threaded and torque together. The tubing string supports the fluid column weight for one half of the total pump cycle during rod lowering after weight transfer has occurred. The rod string reciprocates within the inside area of the tubing string. The standing valve is attached to the tubing string.
FLUID COLUMN: Is the fluid volume being lifted and lowered during the pumping cycle within the tubing string via the rod string. The fluid column dimensions are determined by the distance to the down-hole pump traveling valve multiplied by the inside diameter (area) of the tubing string minus all displaced volume from the rod string. The fluid column is supported and sealed by the traveling valve during lifting portion of pump cycle.
DOWN-HOLE PUMP: Is a single acting positive displacement pump located within the bottom of the total well installation. The three main pump sub-assemblies are the traveling valve, standing valve and working barrel. They all interact during the pump motion cycle. Pump displacement is the volume of media drawn in and pushed out during one pump stroke. Pump diameter along with total distance lifted and lowered of the traveling valve affects pump displacement per pump cycle. Number of reciprocations per time period affects pumping rate.
ANNULUS: Is the differential area or volume difference between the inside diameter of the well casing and the outside diameter of the production tubing string. This volume is where fluid level is measured in height above the pump inlet. This volume is where the down-hole pump draws from during the intake portion of the pump cycle. This volume at the bottom of the installation is where reservoir liquids and gases enter through perforations within the casing. This volume is where natural gas is produced via the well head at the top of the well installation.
PUMP MOTION CYCLE: Is the complete lifting and lowering motion which powers and controls the down-hole pump, also called pump cycle. Lifting the traveling valve creates a void which allows well annulus fluids to be forced past the standing valve. Lowering the traveling valve initially begins the compression phase which forces the standing valve closed, pressurizing the trapped fluids and gases within the working barrel. Once internal pump pressure surpasses fluid column load the traveling valve is forced open. As the open traveling valve continues to be lowered the same trapped fluids and gasses are forced past the now open traveling valve added to the bottom of the fluid column for lifting toward the surface on future pump cycles.
PPRL: Peak Polished Rod Load equals the maximum force required to lift the entire rod string weight plus fluid column weight plus acceleration forces plus all friction forces encountered during the process of lifting the rod string at a given speed and acceleration. PPRL is measured at the polished rod and equals the force required of the surface unit to power and control the down-hole pump at a given pumping rate.
SPM: Strokes per Minute of the polished rod equals the total number of complete rod string lifting and lowering cycles per minute used to power and control the rod pump while producing the well at a given rate. As the SPM changes so does the pumping rate potential of the well. One stroke per minute is equivalent to one complete pump cycle.
POLISHED ROD TRAVEL: Is the distance that the surface unit lifts and lowers the polished rod. This is also the total distance that the rod string at the surface is lifted and lowered. The down-hole pump total travel is completely different, due to the elasticity of the complete rod string and total weights supported by the rod string. As the polished rod travel distance changes so does the pump displacement; one long stroke has larger pump displacement than one short stoke.
DOWN-HOLE PUMP TRAVEL: Is a value affected by many different forces during the course of rod pumping an oil or gas well. The rod string plus tubing string both stretch; the total pump travel is typically less than the polished rod travel, hence under travel. However, depending upon the operating characteristics, total loads, the elasticity of material construction plus pumping SPM the down-hole pump could also experience over travel, meaning the down-hole pump may travel linearly further than the polished rod.
WEIGHT TRANSFER POSITION (WTP): Is the position where the traveling valve ball is hydraulically forced off its seat while traveling downward within the down-hole pump assembly. This position is measured within the polished rod total travel. Weight transfer occurs on the down stroke of the pumping cycle. The weight being transferred is the weight of the fluid column which was previously lifted and supported by the closed traveling valve in the pumping cycle. The weight of the fluid column transfers to the tubing string when the traveling valve is forced open. The weight of the fluid column is then supported by the closed standing valve.
TRAVELING VALVE LEAKAGE: Is also called slippage and equates to the fluid volume that slips or leaks past the traveling valve ball and seat plus the volume that slips or leaks past the plunger and working barrel during the course of rod pump operation. Many factors influence traveling valve leakage or slippage as detailed in the background information. Well production, operating costs and potential failures down-hole are all affected by excessive leakage amounts.
STANDING VALVE LEAKAGE: Is also called slippage and equates to the fluid volume that slips or leaks past the standing valve ball and seat during the course of rod pump operation. Many factors influence standing valve leakage or slippage as detailed in the background information. Well production, operating costs and potential failures down-hole are all affected by excessive leakage amounts.
TOTAL WELL LEAKAGE FACTOR: Is also called total well slippage factor. This is the total leakage or slippage of fluid lifted that does not make it to the surface of the well. It includes traveling valve leakage, standing valve leakage and tubing string leakage total amounts of fluids that slip past and also the rate at which they slip past the sealing surfaces.
BOTTOM-HOLE PRESSURE: Is the hydrostatic head pressure generated by the annulus fluid column of the well bore liquids along with any reservoir formation pressures. The total height of this annulus fluid column is directly proportional to reservoir formation pressure and its ability to flow thru the casing perforations. The greater the bottom-hole pressure; the easier the well bore fluids are pushed into the pump inlet thru the standing valve during the pumping intake cycle. As the bottom-hole pressure diminishes, due to decreased hydrostatic head pressure; so does the ability or motive force for the down-hole pump to fully fill the working barrel during the pumping intake cycle within the time allotted. Bottom-hole pressure is usually a calculated value based on the specific gravity and height of the fluid level within the annulus. Pumping a well dry is the same thing as saying the bottom-hole pressure is very low. In a natural gas well liquid level bottom-hole pressure is directly related to natural gas production. High bottom-hole liquid pressures are synonymous to reduced natural gas production levels due to liquid interference.
FLUID POUND: Is the name given by the oil and gas industry to the damaging effect of operating a rod pumping installation with less than full down-hole pump fillage with fluids. Tremendous stress and strain to the entire well installation is caused by fluid pound. Since the pump is not being adequately filled on every intake stroke daily production rates decline in a direct ratio to pump fillage. Weight transfer position is directly proportional to the effect called fluid pound. The higher the weight transfer position the less likely of causing damage to the well installation due to more complete pump fillage.
GAS POUND: Is similar to fluid pound in which the down-hole pump fillage is comprised of a compressible gas instead of being fully filled with fluids. This tends to not open the traveling valve upon rod string and traveling valve reversal to downward motion due to gas being highly compressive. The trapped gas must be pressurized high enough to overcome the traveling valve load. Similar damaging results occur with gas pound also called gas lock as found with fluid pound. Production also suffers due to lack of fluid being pumped.
PUMP OFF: A phenomenon produced when pump submergence below the fluid column within the annulus is low. Also stated as during the pump intake cycle very little fluid enters the down-hole pump. A pump-off situation will likely increase the gas intake within the rod pump itself, thus reducing the pump efficiency, also called gas pound. Operating coal bed methane gas wells near pump off, produces the greatest amount of natural gas up the annulus due to low bottom-hole liquid pressures. With less force acting downward on the coal bed seam the natural gas is able to de-sorb from the surface of the coal surfaces.
WORKOVER: The process of performing major maintenance or remedial treatments on an oil or gas well. In many cases, workover implies the removal and replacement of the production tubing string or rod string or down-hole pump after a failure was diagnosed. The workover process cannot begin until the well has been killed which severely restricts or limits hydrocarbon transfer from the reservoir formation into the well bore.
KILLING A WELL: To stop a well from flowing or having the ability to flow into the wellbore. Kill procedures typically involve pumping higher density fluids into the wellbore to choke off hydrocarbons from entering the well bore, from the reservoir, and traveling to the surface and escaping out the open hole. This same fluid must be re-pumped out of the well bore and formation to reinstate well production once the workover process is completed. There is no guarantee the well will produce as it did prior to being killed.
Artificial lift is used to produce hydrocarbons from an oil or gas well that does not flow to the surface under its own entrained down-hole energy. Rod pumping is one means of adding surface unit energy for producing the well. This introduced surface energy performs work in the form of artificially lifting a subterranean fluid to the surface through the complete oil or gas well installation. The pumped fluids and gases exit the well bore via the well head thru the fluid production and gas production lines.
As existing oil and gas wells can no longer be economically produced using other forms of artificial lift such as ESP or PCP or gas lift or plunger lift or hydraulic lift it is very common industry practice to switch to rod pumping. Due to lowering bottom-hole pressures the other forms of artificial lift are unable as well as not economically feasible to maintain current production levels. As this occurs over the life of a well installation rod pump means are refitted to the same wells which extend the life of production as well as reducing operating costs and allowing the operating company to continue producing hydrocarbons. Rod pumping benefits are well known and documented. However, they must be operated correctly or costly failures will and do occur, which again hinders production and increases the total cost of producing the hydrocarbons from the oil or gas well.
Rod pumping equipment and parts used to produce oil and gas wells can be categorized into simple working groups. Each group or sub-assembly are constructed of many individual components integrated together all acting in unison. The groups combine together to form the complete oil or gas well installation. The four main working groups are; Surface unit, Polished rod and sucker rod string combination, commonly called the “rod string”, Production tubing string, commonly called the “tubing string” and Down-hole positive displacement pump assembly. Surface unit converts an input energy source of electricity or chemical energy (combustible fuel) into a useable and controllable force that lifts the polished rod against gravity and lowers the polished rod with gravity while producing an oil or gas well via rod pumping.
Polished rod and sucker rod string combination commonly called the rod string. The polished rod is located on top at the surface of the well, reciprocating within the wellhead. The polished rod seals the top of the wellhead. The sucker rods hang beneath the polished rod and extend downward to the bottom of the well. The rod string assembly is an elastic linear system comprised of many pieces of solid steel or “other material” in the form of long slender rods with male ends along with female couplings which are all threaded and torqued together. The individual components, once connected, form one continuous elastic member connecting the surface unit to the down-hole pump assembly. The rod string conveys surface unit energy to the down-hole pump assembly in the lifting and lowering motion profile; hence powering and controlling the down-hole pump operation. It is very common to have devices called rod guides installed on the rod string attempting to reduce friction down-hole.
The polished rod stroke length defines total linear travel distance measured at the surface of the operating well. All polished rod weights suspended below are measured via the polished rod at the surface of the well. The suspended weight; supported by the polished rod is the combined weight of all sucker rods, sucker rod couplings, rod guides and possibly other mechanical type devices plus the fluid column and traveling valve, explained later. In addition to these weights are all mechanical and fluid frictions minus buoyancy effects of the rod string plus all acceleration and deceleration forces during the lifting and lowering process within the column of fluid throughout the entire oil or gas well installation. The rod string supports the fluid column weight while being lifted.
Production tubing string commonly called the tubing string. The tubing string is also an elastic linear system; comprised of long hollow steel or “other material” tube sections threaded and torqued together with couplings and without couplings. They form a continuous linear conduit in which well fluids are produced via the well head at the surface. The rod string reciprocates within the tubing string. Both elastic systems work in unison sharing the weight of the fluids being lifted to the surface at different times during the pumping cycle. The tubing string supports the fluid column weight while being lowered after weight transfer has occurred.
Down-hole positive displacement pump assembly is located at a predetermined depth near the bottom of the oil or gas well and is mounted common with both the rod string and the tubing string. It is a positive displacement device that pumps a volume of fluids based on each mechanical lifting and lowering motion profile of the polished rod.
Total down-hole pump displacement per pump stroke, also called pump cycle is calculated by the effective area of the plunger reciprocating within the inside diameter (area) of the working barrel multiplied by the traveling valve linear movement. Total pump flow rate is multiplied by the total number of complete pump cycles in a given time period. Pump displacement is typically measured and shown in barrels per day. Oilfield barrels are 42 gallon capacity each; a gallon having 231 cubic inches of volume.
The down-hole pump has numerous components; three major groupings or sub-assemblies for this application are; Traveling valve assembly, Working barrel, Standing valve assembly
The traveling valve assembly is a traveling valve attached to the bottom of the rod string and positioned inside the working barrel a ball and seat combination and a plunger. At times, multiple balls and seats are installed in series to further enhance traveling valve robustness. The plunger outside diameter seals against the working barrel inside diameter, aided by the fluid film strength. Varying fluid characteristics down-hole affects this film strength and sealing ability. The dimensional difference of plunger and working barrel is the running clearance of the pump which can change over time by getting damaged with scratches and other forms of damage throughout the operational life of the installation. Both separate entities; balls and seats plus the plunger work together forming the complete traveling valve assembly and referenced as a singular component unless specified differently for the remainder of this document. There are two potential leakage passages past the traveling valve. They are past the balls and seats singularly or plural along with the pump running clearance described previous.
The traveling valve is connected to the bottom of the rod string and reciprocates up and down within the working barrel. As the polished rod is lifted by the surface unit during pump operation a void is created under the traveling valve and within the working barrel. During this lifting motion the fluid column above the traveling valve is trapped, supported and lifted to the surface. The traveling valve seals the bottom of the fluid column weight as the surface unit lifts the polished rod. The traveling valve sealing ability is crucial for proper rod pumping operation. This lifting process is what pulls the fluid column upward producing the well. When the traveling valve leaks during the lifting cycle, the fluid that slips past is not produced at the well head and must be re-pumped back into the fluid column on subsequent pump cycles.
The working barrel is the pump fillage chamber during the lifting portion of the pumping cycle. It is also the high pressure chamber during initial traveling valve lowering portion of the pumping cycle. One complete lifting and lowering pump cycle has a specific volume displacement minus all leakages. The number of complete pump cycles per given time frame creates a flow rate of the well installation as produced at the surface of the well minus what fluids slip past during pumping. When the working barrel allows high pressure fluids to slip past during the pressurization cycle, due to scratches and other damages to the working barrel; those same fluids are not produced at the surface and must be re-pumped in subsequent pumping cycles.
The standing valve assembly is a standing valve anchored to the bottom of the production tubing string via the insert style down-hole pump or tubing style down-hole pump. The standing valve consists of a ball and seat and performs the work of a one way check valve allowing reservoir fluids and gases to enter the working barrel during the intake or lifting portion of the pump cycle and prevents return flow during the pressurization portion and remaining lowering of the pump cycle. As the traveling valve begins to lower, the trapped fluids and gases contained within the working barrel are pressurized, further forcing and sealing the standing valve ball against its seat. As the working barrel pressure level rises and then exceeds the force holding the traveling valve shut; the traveling valve ball is forced open off its seat. This position, measured at the polished rod, is called weight transfer position. WTP is used extensively throughout this document; it is defined as weight transfer position. After the traveling valve ball is forced open and while continuing downward; the newly trapped and pressurized fluid and gas volume within the working barrel enters the fluid column to be lifted to the surface on subsequent pump cycles. When the standing valve allows high pressure fluids and gases to slip past during the pressurization cycle and during the remaining lowering cycle while supporting the fluid column weight those same fluids and gases are not produced at the surface and must be re-pumped in subsequent pumping cycles.
By altering the time allotted to complete one full pump cycle you alter the pump flow rate within the same time allotted. By altering the polished rod stroke hence effecting total traveling valve distance within the working barrel you alter the pump displacement volume per stroke. By adding dwell time while the polished rod is fully lifted and held stationary for a time amount you allow the down-hole pump additional time to fill the working barrel. By reducing total polished rod travel; pump displacement is reduced the same proportional amount, not counting rod string stretch. With reduced displacement down-hole a higher pump fillage percentage of fluid volume may also be increased when fillage percentage is compared to full polished rod travel. If time dwell and stroke reduction are both introduced additional pump fillage assistance and total fillage percentage is increased. By altering the complete number of strokes per given time frame you affect the pump total flow rate within the given time frame. Changing these pump cycle parameters all affect the total pumping flow rate of the complete well installation within a given amount of time. If one could vary these pump cycle operating parameters based on a detectable and known condition one could alter production of the well when the reservoir formation fluid inflow volume to the annulus changes over time either increasing or decreasing.
Simple Down-Hole Pump Operation
Surface unit begins to lift polished rod
Rod string elongates or stretches
Rod string begins lifting traveling valve
As the traveling valve is lifted; the ball is forced into the seat and closes from the weight of the fluid column being lifted
The traveling valve seals the bottom of the fluid column being lifted minus the fluid which slips past
The fluid column weight is fully supported by the rod string and closed traveling valve
The weight of the fluid column has transferred from the tubing string to the rod string during the lifting portion of the pump cycle
The polished rod is now supporting the full weight of the fluid column and all steel being lifted by the surface unit
As the traveling valve is lifted; a void or low pressure zone is created within the working barrel
This is the down-hole pump intake stroke portion of the pumping cycle
Standing valve is forced open by the pressure differential caused by height of the fluid level above the pump inlet also called bottom-hole pressure; along with any reservoir formation pressurized fluids and gases available to enter the void just created within the working barrel
Surface unit reaches end of lifting portion of pump cycle; whereby polished rod is fully lifted
Surface unit begins lowering the polished rod
Rod string shortens and compresses due to resistance of movement downward
Rod string begins lowering the traveling valve and previously lifted fluid column
Traveling valve pushes down on fluid and gases within the working barrel and forcing the standing valve closed
Traveling valve continues downward further pressurizing the now trapped fluids and gases within the working barrel and standing valve
Rod string continues shortening and compressing caused by additional resistance during pressurization of trapped fluids and gases
When sufficient working barrel pressure is generated, the traveling valve ball is forced off its seat and opens
When the ball is forced open, off its seat, the fluid column weight transfers back to the tubing string; now fully supported by the closed standing valve, this is weight transfer position (WTP)
Polished rod continues lowering the traveling valve; forcing the trapped fluids and gases within the working barrel, to flow past the now open traveling valve and enter the bottom of the fluid column volume
Surface unit reaches end of down stroke, whereby the polished rod is fully lowered
Pumping cycle is repeated until surface unit is shut off or failure occurs
All fluids which slip past during the pumping cycle are not produced at the well head and must be re-pumped on future pump cycles
In addition to crude oil wells, down-hole pumps are used for the deliquifaction of coal bed methane natural gas wells. Methane gas commonly found in a coal bed seam or coal bed; tends to adhere to the local surface of the coal itself while under hydrostatic head pressure caused by the weight of fluids found in the earth pushing down on the coal bed seam. Due to Pascal's Law the pressure is equal in all directions and the amount of force on the coal bed is directly related to the total height and volume of the fluid acting upon the coal bed within the earth. As the coal beds are submerged in these ground fluids, the force acting on the coal via hydraulic pressure causes the methane gas to adhere to the coal itself according to the principle of adsorption. As the rod pumping installation removes this down acting force via lifting these fluids off the coal bed, the hydraulic pressure is temporarily decreased, which allows the natural gas to be released off the coal surfaces.
During rod pumping down-hole pressure is reduced causing the natural gas and associated fluids to flow or travel towards this lower pressure zone via cleats or cracks found within the coal bed seams. These fluids and gases enter the well bore annulus through perforations in the casing below ground and travel up the annulus of the well installation. Some of the gas is conveyed within the ground fluids via the tubing string as both media enter together thru the down-hole pump installation. Typically natural gas flows to the surface of the wellhead via the annulus. The fluids that travel up the annulus help to fill the down-hole pump during the pump intake cycle. The height of these fluids within the lower portion of the annulus is measured via other methods. This hydrostatic head pressure acting on the pump intake is the down-hole pressure. The higher the vertical column is within the annulus the greater the down-hole pressure and vice versa.
As the coal bed is deliquified, the surrounding ground fluids adjacent to the well tend to refill the voids within the coal beds. Ground fluid levels vary for many reasons and the actual re-charging of the same coal bed could be to previous levels or higher or lower without human control over time. As forces reach levels of equilibrium, due to the inflow of the ground fluids, hydraulic pressure again tends to retain the remaining natural gas to the coal surfaces as described above, again choking off natural gas production. However, if the rod pumping installation continues to remove the fluids at a rate that exceeds the ability of the earth to refill the coal bed, the down-hole pressure will continue to decrease, enabling more of the gas to be released and produced thru the well installation.
When the rod pump installation continues to remove ground fluids, at some point the coal bed may be effectively pumped off, not able to fill the working barrel with fluids during the intake cycle, if at least temporarily. Extended operating time of the rod pump equipment without sufficient amounts of down-hole fluid fillage of the working barrel will cause serious damage to the total well installation. These damaging effects are typically called fluid pound. If the surface unit has a method of detecting the varying well inflow conditions and autonomously matching the polished rod motion profile with reservoir inflow fluid rates via adding dwell time at the top of the intake cycle or decreasing the total polished rod travel or combinations of both as well as changing total strokes per minutes, without shutting off, maximum daily gas production is achieved while minimizing the damaging effects of fluid pound by always maintaining full working barrel volumes of fluids.
Mechanically driven and counterbalanced surface units are unable to autonomously adapt to these changing down-hole inflow conditions. They are not able to add dwell time at the top of the pump intake stroke, nor are they able to adjust their total polished rod travel autonomously while operating. The best they achieve is to turn themselves on and off when fitted with run timers. The surface unit operates for an adjustable set period of time on, and then rests idle off for an adjustable set period of time, while the coal bed refills with ground fluids. The equipment will therefore cycle on and off to remove the ground liquids and then allow the liquid level to refill. If they are fitted with pump off controllers and variable speed drives they can adjust between a minimum and maximum strokes per minute, but not without short comings. Rod heavy conditions produce massive over running inertial torque values which must be dissipated into heat or regenerated with additional hardware further increasing cost of installation. If the inertial torque values are not managed variable speed drive high voltage bus faults occur and drive internal overload limiters shut the drive down. Further adding operating costs for dispatching a human to cycle the power on and off at the location to reset the drive plus lost production. During the time the coal bed methane well sits idle, the rod pump equipment not operating, ground fluids refill the coal bed seams, raising bottom-hole pressures, choking off the natural gas produced from the well.
During off time from the previous paragraph and having excessive traveling valve or production tubing string leakage; reduces daily production while increasing daily operating costs. These slippages allow previously lifted fluids to fall back into the well which must then be re-pumped when the surface unit begins operation again. Standing valve leakage during the pressurization cycle; reduces the volume of fluids pumped by allowing working barrel pressurized fluids to flow back into the annulus and reservoir formation instead of forcing the traveling valve open and adding these trapped liquids to the fluid column above the traveling valve. All three leakage factors directly affect total well revenue by reducing daily hydrocarbon production and increasing daily operating costs.
Down-hole pump leakage also called pump slippage or slippage factor or slippage rate affects the daily fluid or gas production volumes and the total well system efficiency, hence costs of operating. Total well slippage factors are made up of individual leakage rates from numerous sub-assemblies within the total well installation. Some of these leakage sources or slippage factors are caused by the traveling valve, standing valve and tubing string. The tubing string is included with the traveling valve test. Performing both traveling and standing valve test methods are required for a complete well slippage test result.
Pump running clearance is the inside dimension of the working barrel versus the outside dimension of the traveling valve plunger. This dimensional difference allows a path of fluid leakage from the fluid column being lifted while supported by the traveling valve ball and seat combination plus the plunger. As fluid slips past, it travels downward back into the working barrel of the down-hole pump. Running clearances are specified at time of ordering or rebuilding of the pump. Running clearances are required for correct pump operation and vary over time of operation and conditions down-hole. Running clearances increase as the life of the down-hole pump increases or gets damaged thru the course of normal operation at varying degrees over time. Tracking these slippage factors over time would allow an operating company to track well performances and allow advanced knowledge for scheduling of workover services (repair) for the total well installation.
Total well slippage or leakage factor is further influenced by numerous well characteristics. Some, yet not limited to, are diameter of pump, length of pump stroke, length of plunger, depth of pump setting, API gravity, type of fluids, water percentage of fluids lifted, sand content of fluids lifted, other contaminants of fluids lifted, life (number of pump cycles) of down-hole pump, condition of down-hole pump and any leakage paths in the production tubing to name a few. In addition to the mechanical reasons stated; time the surface unit is not actively pumping the well provides additional time for the tubing string fluid column to leak into the annulus or past the traveling valve or reservoir formation via the standing valve.
As pumps are set deeper; the fluid column exerts higher hydrostatic head pressures at the bottom of the well increasing pump slippage or leakage factors thru these leak points or pump clearances. As pumps begin to wear; the total slippage or leakage factor increases over time and number of pumping cycles. All of the above conditions directly affect the daily production of the well and hence the total operating cost of the entire oil or gas well installation.
Dynamometry is very common prior art which is accepted and proven within the oil and gas industry today. The technique measures the polished rod motion dynamics in real time while the testing and recording equipment is installed and connected to a rod pumping installation. The well testing process requires specialized equipment and related software and specific human knowledge plus proper step by step human intervention and procedures for correct well testing results. When performed correctly, any rod pumped oil or gas well installation can be verified for proper operation or diagnosed in determining what is not correct.
The process measures and records and analyzes the polished rod motion profile including; weight supported and travel direction and travel velocity and position and acceleration and deceleration forces. In addition electric motor voltage and current readings are simultaneously measured and recorded and over laid on top of the polished rod motion characteristics.
The monitored and translated feedbacks are compiled and output as the surface unit card also called dynamometer surface card or just surface card. It depicts the actual amount of real work during the lifting and lowering motion profile completed at the surface of the complete oil or gas well installation.
The safest way to perform dynamometer well testing includes shutting down the surface unit to install the testing hardware feedback devices. Once the well testing hardware is installed, the surface unit is re-started and test data is recorded. Different well tests have varying test procedures or test processes. After a designated run time or number of pumping cycles for the duration of the well tests; the surface unit should be shut down for safely removing the well testing hardware. Sometimes the testing equipment is installed and removed while the surface unit is running but that is not typically the safest operating practice.
Basic Steps for Traveling Valve Test as Performed Using a Dynamometer
The technician performing the slippage test must enter all well bore data into the tailored computer and software of the dynamometer testing system in order to produce accurate testing results
If the entered information is not correct it will directly affect the testing results
All remaining steps are human managed and performed
Surface unit is shut off
Dynamometer testing system feedback devices are installed
Surface unit is re-started and correct feedback operation is verified
It is very common practice to record several minutes of actual pumping operations for analyzing the polished rod motion dynamics as well as determining where WTP is occurring
While the polished rod is being lifted
Surface unit is switched off
Simultaneously the human sets the mechanical braking device
Not setting the brake quick enough may allow the rod string to fall downward possibly damaging the well installation
Polished rod is held stationary by the braking device, if it moves the test results will be affected
The manually installed dynamometer testing system monitors the polished rod motion profile along with supported weight for the designated time amount
During this time amount the polished rod supported weight decays at a rate measured over time of the test
Based on the previously entered well bore details a leakage rate is determined of the traveling valve
All testing feedback devices are removed
Surface unit is re-started or a standing valve test is performed next
If the standing valve test is required weight transfer position must be determined prior to performing standing valve test
Standing Valve Test as Performed Using a Dynamometer
The same well bore data is used for the standing valve test.
Dynamometer testing system feedback devices are still installed and operating correctly
Surface unit is pumping the well
Weight transfer position is determined with the data collected previously
All remaining steps are human managed and performed
It is very common practice to record several minutes of actual pumping operations for analyzing the polished rod motion dynamics plus double checking where WTP is occurring
While the polished rod is being lowered
After weight transfer position has occurred which is either a guess or information interpreted during a previous monitoring run is used
Turn off the surface unit
Simultaneously setting the mechanical brake
Not setting the brake quick enough may allow the rod string to fall downward possibly damaging the well installation
Polished rod is held stationary by the braking device, if it moves the test results will be affected
The manually installed dynamometer testing system monitors the polished rod motion profile and supported weight for the designated time amount
During this time amount the polished rod supported weight increases at a rate measured and recorded over time of the test
Based on the previously entered well bore details a leakage rate is determined of the standing valve
Surface unit is switched off
All testing feedback devices are removed
Surface unit is restarted
Specialized hardware and software along with specific manual techniques and procedures are both required to perform traveling valve and standing valve dynamometer card well testing, both are tested per well for a complete test. Having these tools, plus a technician with the training required is essential to properly evaluate the entire rod pumping installation for operating efficiency and for testing the leakage or slippage rates of the entire well along with diagnosing well short comings which could lead to pending equipment failures.
A typical piece of testing hardware is a polished rod transducer; which attaches to the outside diameter of the polished rod. Another transducer style is installed in a way that fully supports the rod string weight. Both devices when properly installed measure working conditions, also called polished rod motion profiles, in both directions of travel. The measurements include polished rod supported weight in pounds force, position, total distance traveled, direction of travel, velocity, acceleration and deceleration forces. All these motion parameters are monitored, analyzed and recorded both while lifting and lowering over the course of testing time while the surface unit is operating and the testing system is installed and recording data.
English units of measure are typically pounds force, both in static and dynamic weights or loads along with acceleration and deceleration gravity (G) forces due to movement over time of actual supported loads. Time is measured in seconds to a very fine degree having capabilities down to 0.001 second loop scan where all feedbacks are recorded every loop scan and collated independently for complete motion profile evaluation. Total polished rod travel distance measured over entire pump cycle up and down typically displayed as inches. The metric system is available as well. Units of force and distance are both able to resolve to 0.1 decimal place.
Electric motor data is observed and recorded during same testing process. Typical electric motor hardware feedbacks are voltage and current measurement. They are installed on the electric motor leads common to the supply power grid. Other electric parameter readings could be lagging and leading power factors and kilowatt use-ages. These measurements could be both instantaneous and per hour among others and able to resolve to 0.1 decimal place.
All feedback devices are connected via wires or cables to a tailored monitoring and recording computer device with purpose built software. The real time data feedback streams are time stamped per scan segment and all data input channels are read, stored, analyzed and manipulated simultaneously. All feedback data streams are available for studying the measured values as they occurred during the time the hardware is installed on the surface unit but only during the testing process is data available. After the well testing is completed the equipment is removed and normal operation is reinstated without any of the fine degree of measurements available.
Due to the difficulty and expense required to measure the actual conditions in the bottom of the well installation where the down-hole pump is installed, a mathematical formula commonly called the wave equation is used to infer down-hole pump operating conditions. The wave equation is also a prior art form and the oil and gas industry accepted standard. The wave equation has numerous forms, often modified and or managed differently by others in performing certain tasks as required by others.
The wave equation is a complex mathematical process which allows for a semi-predicable solved output when a known bi-directional force (lifting and lowering) in an applied direction plus applied amplitude plus rate of change is introduced to the polished rod at the surface. The resulting inferred output of the non-supported opposite end of the elastic rod string is then a calculated output value. These calculated values include but not limited to traveling valve distance both up and down. Forces applied to the traveling valve both up and down. Both of these calculated value streams are dynamic and time based during the rod pumping cycles. Stated another way is that the bottom of the rod string lags behind the top of the rod string for both lifting and lowering the polished rod and because it lags behind pump under travel and over travel occurs where the traveling valve does not move the same amount as the polished rod and the wave equation attempts to predict down-hole conditions such as pump fillage percentage among others.
In order to produce the requested accurate output of the wave equation; complete and accurate well bore diagram information with all respective rod string, tubing string, down-hole pump component sizes, lengths, diameters, types, configurations, pump setting depths, fluid type specifics, water percentage, gas percentage, deviated, directional or straight well type configurations all must be entered in the above mentioned tailored computer and software system.
The inferred output is typically called the “down-hole pump card”, “dynamometer down-hole pump card”, or just the “pump card”. Un-like the surface unit card, the down-hole pump card depicts the inferred amount of real work completed at the bottom of the oil or gas well total installation based on the wave equation in addition to all user input data of the well bore diagram.
Through the process of well testing using a dynamometer; a person skilled in the art of dynamometry can properly diagnose and determine the reason why a well produces poorly or mechanically fails often. One can also verify correct operation in a normal producing well as a means of routine system operating inspection or record keeping or tracking for the life of a well installation, which could trend a pattern of mechanical failures versus production or other statistical data. The above described well testing data or process is only available while and during the testing hardware and software and trained technician are present and running on the specific well installation. Dynamometer testing equipment is not designed for permanent well installation.
Total well installation operating deficiencies represented could be yet not limited to: reduced down-hole pump fillage capacity or rates of fillage, pumped off well condition, parted rod string down hole, out of balance conditions in the surface unit, over or under loaded surface units, production tubing leakage, traveling valve leakage, standing valve leakage among numerous other production hindering well attributes. All of which directly impact the operating company's total cost of producing an oil or gas well.
For dynamometer testing, technician intervention is required for performing the required mechanical and electrical connections plus proper procedural operation such as stopping and setting the brake of the surface unit during the correct portion of the pumping cycle and for releasing the brake and re-starting the surface unit correctly to prevent damaging the surface unit upon completion of leakage or slippage tests. All well specifics unique to each well site must be entered into the proprietary software for performing well testing and producing the surface card plus the inferred down-hole pump card. Substantial technician effort and coordination in correct sequences of operation plus safety procedures and resultant costs are required for a well to be properly and safely tested using a dynamometer. Due to these costs it is common that wells are only tested when a problem is suspected or production rate declines or stops due to a failure of some type.
The well testing data derived from a dynamometer test is not currently nor typically, used to autonomously adjust the existing surface units used today. These surface units include both mechanically counterbalanced designs and hydraulically operated designs with or without high pressure nitrogen gas used for counterbalance. Changing total polished rod stroke travel or changing the mechanical counterbalance weights requires physical alterations of the surface unit plus heavy lifting equipment and adding or removing heavy counterbalance masses plus more manual human effort. If final drive speed reductions are desired to affect global strokes per minute, mechanical hardware reconfiguring of the surface unit is required by changing the running diameter of the driven sheave or driver sheave or both. These additional human produced changes could also include adding or subtracting nitrogen gas pressure volume for changing the counterbalance effect in some existing hydraulically powered surface units today.
The pumping speed of surface units predominantly in use today typically can only be varied between a designated minimum and maximum speed or be varied by varying run time versus off time of the surface unit. These simple variables are accomplished with additional control or powering equipment such as pump off controllers plus variable speed drives with or without over-running load reactors or just simple run timers. In the case of an installed pump off controller plus variable speed drive with or without over-running load reactors; gross load changes are able to be determined using many different means of prior art in the form of load plus designated positions plus time and the varying time amounts from cycle to cycle and current sensors while the surface unit is operating.
The foregoing do not provide for dynamometer accuracy but rather are gross means of detecting and preventing the damaging effect of fluid pound as reservoir down-hole conditions change.
The following are objectives of the present invention
Autonomously perform traveling valve or standing valve slippage tests
Autonomously transmit well slippage or leakage factor test results locally or externally
Autonomously detect weight transfer position and self adapt to changing well in flow conditions or lowering fluid levels within the annulus also caused by varying reservoir inflow conditions
Using the slippage test data with other data to self adjust rod pumping motion profile by adding a dwell time at the top of the polished rod travel (pump intake stroke) based upon weight transfer position trending lower from previous pump cycle or average of previous pumping cycles. The added time value allows additional time to fill the working barrel during the pump intake duration
Using the slippage test data with other data to self adjust rod pumping motion profile by decreasing or removing a dwell time at the top of the pump intake stroke based upon weight transfer position trending higher on previous pump cycle or average of previous pumping cycles
Using the slippage test data with other data to self adjust the running counterbalance values so lifting and lowering the rod string is always in balance, hence not over loading the surface unit or causing high voltage faults or wasting electrical energy thru resistor banks when variable speed drives are used to regulate total strokes per minute
Using the slippage test data with other data to self adjust by reducing the polished rod total travel amount based on weight transfer position from previous or average of previous pump strokes trending lower; this allows for complete working barrel tillage which reduces the damaging effects of fluid pound
Using the slippage test data with other data to self adjust by increasing towards maximum polished rod total travel amount based on weight transfer position from previous or average of previous pump strokes trending higher; thus allowing more well production
Using the slippage test data with other data to self adjust the top and bottom direction-change position to accommodate worn down-hole pump conditions as determined by the slippage tests
Using the slippage test data with other data to self adjust the acceleration and deceleration rates prior and after direction-change occurs to reduce the stress and strain on the rod string